Athletes cover great distances. In order to reduce the energy required for that, the weight, particularly also of sports shoes, is desired to be reduced. A reduced weight requires less force during accelerations, for example, and thus reduces the strain on the muscles of a player or runner.
An essential factor determining the weight of a shoe is the material of the shoe upper, which determines the weight of the sports shoe in combination with the sole. Indeed, the weight of the shoe upper may in principle be reduced by simply using less material, for example, by reducing material thicknesses. On the other hand, the shoe upper has to fulfill high quality standards, for example, with respect to abrasion resistance, flexibility, lamination, or stability. In order to fulfill these different requirements, use of multilayer or composite materials may be desirable.
Different shoe uppers with multilayer materials are known in the prior art, such as the combination of a textile material with a plastic layer. For example, DE 10 2009 028 627 A1 describes a shoe upper with textile materials, for example a nylon fabric. The textile material may be additionally enhanced on the outside and/or the inside by an appropriate coating, for example by a transparent layer made from thermoplastic polyurethane (“TPU”), which is glued onto the textile material with hot glue.
US 2010/0037483 A1 describes a material for a shoe upper in which fibers are arranged between a base layer and a cover layer for which a thermoplastic polymer material, for example TPU, may be used. Similar materials for a composite material for a shoe upper are described in U.S. Pat. No. 3,397,418, U.S. Pat. No. 6,558,784 B1, and U.S. Pat. No. 7,941,942 B2. Furthermore, US 2007/0199210 A1 describes a material in which a middle layer is arranged between a first layer and a second layer. The middle layer is a textile fabric, and the first and the second layers may be made from a foam material, for example, ethylene-vinyl acetate (“EVA”) foam.
US 2011/0088282 A1 describes a composite material for a shoe upper having an outer skin layer and a substrate layer, between which a fabric layer is arranged. The substrate layer may comprise a laminate with a first material layer (e.g. leather) and a second material layer (e.g. TPU). The outer skin layer may consist of TPU or a multilayer material, for example, an outer layer from thermoset polyurethane (“PUR”) or TPU, and an inner layer from TPU.
However, the use of TPU has various disadvantages. For example, TPU discolors when exposed to light (yellowing). This property impairs the quality of the material. Further, TPU is susceptible to hydrolysis. This characteristic may result in delamination of layers of material. Furthermore, a good bond of TPU with other materials usually requires an additional adhesive layer, which often results in higher weight, delamination of material layers, or impaired material quality. In general, TPU has certain mechanical material properties that may be undesirable, depending on the application. For example, TPU is stiff and inflexible due to the required adhesive layer.
In contrast, omitting the adhesive layer with TPU may also lead to problems. For example, a bond between TPU and PUR without an adhesive layer does not create a sufficient bond, since different materials are used.
It is therefore desirable to provide a composite material for a shoe upper for sports shoes to overcome at least parts of the specified disadvantages of the prior art and thus to provide a shoe that is particularly light without a loss of quality.